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    <title>Document</title>
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    <script>
        {
            var c = 0 ;
            let a = 10 ; 
            const b = 50 ;
            console.log(c);
            console.log(b);
            console.log(a);
        }//js代码可以用一个大括号将一段代码括起来，表示一个域
        console.log(c);
        console.log(b);
        console.log(a);
    </script>
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